1. Field of the Invention
The present invention relates to a magnetic levitation control apparatus, such as a magnetic bearing or a magnetic levitation damping apparatus, and more particularly to a control apparatus for holding a levitated body having a magnetic body in a predetermined position in a non-contact manner by controlling the magnetic attraction of electromagnets.
2. Description of the Related Art
In the magnetic levitation control apparatus, a detector for detecting the position of the levitated body and a feedback control unit for controlling the magnetic attraction of the electromagnets based on the detected positional information are generally required and indispensable as a mechanism for stably holding the levitated body in a non-contact manner. One example of the construction is shown in FIG. 1. Position detection sensors 22a, 22b are a conventional sensor called a xe2x80x9cmagnetic induction systemxe2x80x9d, wherein the position of the levitated body is detected in a non-contact manner by a change in inductance, and, together with balancing resistors 23a, 23b and a differential amplifier 26, constitute a bridge circuit. A carrier signal generated by an oscillator 25 and a buffer amplifier 24 is added to the bridge circuit. Upon the occurrence of unbalance of the inductance between the sensors 22a, 22b due to the displacement of the levitated body 21 from the reference position, a sensor signal having an amplitude substantially proportional to the displacement appears in the differential amplifier 26. The sensor signal contains, in a superimposed state, a noise signal included by magnetic coupling between the sensor and the electromagnets or between the sensor and other electromagnetic actuator. Therefore, a band-pass filter 27 with the frequency of the carrier signal being the central frequency is generally provided to remove the noise component.
In FIG. 1, in order to remove the carrier frequency component to extract the amplitude component only, the sensor signal passed through the band pass filter 27 is processed in a detection circuit 28, and sent as a displacement signal to a controller 29. On the other hand, electromagnets 35a, 35b are excited by a voltage control type current source comprised of differential power amplifiers 32a, 32b and current detecting resistors 34a, 34b. In general, an adder 30 and a subtracter 31 are used to supply, to the electromagnets 35a, 35b, a superimposed current composed of a bias current corresponding to the voltage of a direct-current power supply 33 and a control current corresponding to the control signal of the controller 29. This creates a difference in magnetic attraction between the electromagnets 35a, 35b. This difference cancelles the force of disturbance and the gravitational force acting on the levitated body 21, whereby the levitated body 21 is held at a predetermined levitation position.
In the above-described feedback control unit, in recent years, a large number of examples of the construction of digital controller are also found in the field of the magnetic levitation control apparatus, from the viewpoints of easiness in implementation on hardware of the control law at the time of design of a control system and the flexibility of a change in control law. As compared with the analog controller, however, the digital controller is disadvantageous in that extra delay elements in frequency response, such as a hold element in the discretization of displacement signal and a waste time element dependent upon the operation time, are included in the feedback control unit. For this reason, when feedback control with quick response is required, for example, in magnetic bearings or magnetic levitation damping apparatuses, hardware having good frequency characteristics should be constructed in the detector for detecting the position of the levitated body and the section for controlling the attraction of the electromagnets.
In recent magnetic levitation control apparatuses, attention is being drawn to the application of a sensorless magnetic levitation system wherein the levitation position is detected using electromagnets per se aiming at the utilization of the apparatus within a vacuum environment by taking advantage of non-contact bearing and a reduction in cost and a reduction in size of the whole apparatus. In this sensorless magnetic levitation system, a carrier signal is directly supplied to the electromagnets 35a, 35b without use of the sensors 22a, 22b shown in FIG. 1 in the detection of the position of the levitated body, whereby the position of the levitated body is detected based on a change in inductance of the electromagnets thereof.
This magnetic levitation system is particularly disadvantageous in that a deterioration in frequency characteristics in the position detector due to a lowering in the frequency of the carrier signal is unavoidable. This suggests that, in the magnetic levitation control apparatus utilized within the vacuum environment, from the viewpoint of vacuum contamination, a thin metallic pressure bulkhead is preferably provided between the sensor and the levitated body. In this case, however, as the frequency of the carrier signal increases, the loss in this portion increases. Therefore, the frequency of the carrier signal is preferably as low as possible. This necessarily requires the use of a carrier signal having a low frequency. The use of the carrier signal having a low frequency results in interference with the frequency band for controlling the position of the levitated body. Therefore, in order to further develop the application of the magnetic levitation technique while utilizing the advantage of the digital controller, it is necessary to take some measure for preventing the deterioration in frequency response of the feedback control unit due to a lowering in carrier frequency of the position detecting sensor.
Under the above circumferences, the present invention has been made, and it is an object of the present invention to provide a magnetic levitation control apparatus that applies self-sensing control, which utilizes electromagnets for controlling the levitation position of the levitated body, to a position detecting system for detecting the position of the levitated body and can achieve a lowering in carrier frequency without a deterioration in frequency response characteristics of the feedback control unit.
It is another object of the present invention to provide a magnetic levitation control apparatus which comprises a combination of the digital controller with the self-sensing control and can realize good controllability.
In order to attain the above objects, according to an aspect of the present invention, there is provided a magnetic levitation control apparatus comprising: a pair of electromagnets for holding a levitated body having a magnetic body in the levitated state, the pair of electromagnets being positioned opposite to each other in such a manner that the point of application of the electromagnetic attraction in the electromagnets conforms to the point of a position detected using the electromagnets as a position sensor. A signal source supplies a voltage signal of a frequency on a level such that enables the electromagnets to function as the position sensor, wherein a control voltage signal for controlling the magnetic attraction of the electromagnets is superimposed on the voltage signal. A circuit differentially supplies the voltage signal to the pair of electromagnets to form a position signal of the levitated body from an add signal of currents respectively from the electromagnets, and a circuit detects a control current of the electromagnets from a subtraction signal of currents respectively from the electromagnets. A controller generates a control voltage signal of the electromagnets from the position signal of the levitated body and, in addition, corrects the detected position signal from the detected control current of the electromagnets.
According to the magnetic levitation control apparatus of the present invention, lowering the carrier (signal source) frequency in the detection of the position of the levitated body can be realized by synchronous detection operation synchronous with the carrier (signal source) signal through the controller. Therefore, even though the frequency band in the operation for detecting the position of the levitated body overlaps with the frequency band in the operation for controlling the levitation position of the levitated body, both operations can be successfully performed without any problem and the position of the levitated body with a relatively high frequency can be detected using even a low frequency signal source.
The self-synchronous detection system does not require a filter for removing the carrier frequency component in the detection of amplitude, and, thus, in principle, the delay of the response of the feedback control system is governed by the sample/hold element and the delay element inherent in the controller used. That is, the discrete value control system can be designed with ignoring the delay of response in the detection of position. Further, the frequency of the carrier signal is necessarily limited to several kHz in consideration of the performance of commercially available digital signal processor boards. This is suitable for self-sensing control.
Since the position detection of the levitated body is carried out by means of electromagnets used in the control of levitation position of the levitated body, the point of application of the magnetic attraction conforms to the point of the detected position. This makes it possible to perform accurate control and, at the same time, to correct the position signal detected from the control current of the electromagnets while taking into consideration, for example, the influence of the magnetic saturation. Therefore, the accuracy of the detection of the position of the levitated body can be enhanced. In general, according to the present invention, since individual sensors for the detection of position are not required, a simple structure can be provided, and a high level of levitation position control can be realized using a simple construction of control circuit.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.